In our case, Auggie's costume for halloween is very bulky (it's a truck) and we've realized he can't really wear it to a little party we're going to or he'll have a hard time running around indoors. So we're saving the big costume for Wednesday and improvising for the party.
Here are some of my favourite last-minute (or just easy) costume ideas and tutorials as well as some free printable masks, all from pinterest.
Printable masks (lion, elephant and owl) from sweetlittleworld:
Jet-pack tutorial (detail) from doodlecraft:
Chef onesie for babies and other easy baby costume ideas from giggle:
Paper bag masks (lion, fox, owl and monkey) from handmade charlotte
Felt bat mask tutorial from living with punks
Pretty printable cat carnivale mask from mer mag via babble